1. Field of the Invention
The present invention relates to valves for use in oil and gas wells as well as other surface flow controls. More particularly, the invention relates to a critical flow valve that provides an annular flow path for gas or other compressible fluid medium and has adjustable throat area to increase or decrease the critical volume rate of gas flowing through the valve due to relative changes of the throat area. The throat area may either be adjustable automatically or manually depending on the purpose of utilization of the valve.
2. Description of the Related Art
The area of application of the present invention is not limited to the recovery of hydrocarbon fluids, but some typical embodiments are chosen in terms of oilfield application for example.
To obtain hydrocarbon fluids from an earth formation, a wellbore is drilled into the earth to intersect an area of interest within a formation. The wellbore may then be “completed” by inserting casing within the wellbore and setting the casing therein using cement. In the alternative, the wellbore may remain uncased (an “open hole” wellbore), or may be only partially cased. Regardless of the form of the wellbore, production tubing is typically run into the wellbore primarily to convey production fluid (e.g., hydrocarbon fluid, which may also include water and gas) from the area of interest within the wellbore to the surface of the wellbore. In an alternative, the annulus formed by tubing and casing may be used as the production stream.
Often, pressure within the wellbore is insufficient to cause the production fluid to naturally rise through the production string to the surface of the wellbore. Thus, to carry the production fluid from the area of interest within the wellbore to the surface of the wellbore, artificial lift means is sometimes necessary. Gas lift and steam injection are examples of artificial lift means for increasing production of oil and gas from a wellbore.
Gas lift systems are often the preferred artificial lifting systems because fewer moving parts exist during the operation of the gas lift systems than during the operation of other types of artificial lift systems, like sucker rod lift systems. Moreover, because no sucker rod is required to operate the gas lift system, gas lift systems are usable in offshore wells having subsurface safety valves that would interfere with a sucker rod.
FIG. 1 is illustrative of a gas lift operation producing production fluid through tubing while the injection gas flows through an annulus formed between tubing and casing. In other applications, the gas may be injected through tubing while the production stream flows through the annulus. Production fluid P flows from formation 55 into wellbore 30 through perforations 60 formed in casing 25. From the wellbore, the production fluid P flows into the production tubing 20 and to a wellhead 35 for collection. When it is desired to lift the production fluid P with gas G, compressed gas G is introduced into the annulus 15. Any of the gas lift valves 45 which are in the open position allow the gas G to flow into the production tubing 20 through an opening in the gas lift mandrel 40 to lift the production fluid P to the surface of the wellbore 30.
Gas lift valves typically include a restriction that is intended to control the flow of gas entering the production string. Choked flow or “critical” flow relates to a condition in which the flow of gas through a nozzle reaches its maximum flow rate with the local flow velocity equivalent to the speed of sound (sonic velocity) at the throat. At that point and over some range of pressure differentials between the near upstream and downstream of the valve, the flow rate of gas will remain stable and unchanged in spite of variances in the relative pressure. For example, for the simple orifice nozzle valve, the flow rate will be stable and limited once an absolute pressure ratio is less than 0.528. In more practical terms, a volume of gas passing through a valve at critical flow will not be so affected by pressure variations between the upstream and downstream sides of the valve. The disadvantage of using a simple orifice nozzle valve is that the pressure ratio across the valve is well above the required critical pressure ratio, and therefore the critical flow condition is unlikely to occur for usual gas lift operation.
More recently, gas lift valves have included a venturi in place of a simple orifice. A typical venturi includes an inlet portion, a throat portion and a diffuser portion. With a venturi, losses of energy in injection gas flow are significantly smaller and a significant pressure recovery occurs along the diffuser of the venturi. As a consequence, the critical flow condition is easily achievable at the pressure ratio of 0.9 or lower keeping a constant flow rate through the valve for fluctuating pressure environment.
U.S. Pat. No. 6,568,473 teaches a venturi with an annular flow area formed between the interior of a valve housing and an exterior of a plug. The '473 patent is incorporated by reference in its entirety herein. Like a more typical venturi, the annular flow path includes an inlet, throat and diffuser portion. In the '473 patent, the plug portion is movable relative to the housing portion in order to close the valve in the event the flow of injection gas does not have enough pressure to overcome the combined force from valve spring and production fluid pressure.
While the '473 patent provides an effective venturi design, it falls short of solving some of the continuing problems associated with gas lift valves. One of the problems is “heading”. Heading is a periodic and unstable flow phenomenon in both the production stream and gas injection sides leading to a dramatic reduction of fluid production and excessive injection gas consumption. Heading occurs when a pressure differential between the injection gas and the production fluid changes due to a transient fluctuation in well conditions. This temporary change of production pressure may come about as a result of an increase in a production gas/oil ratio in the well. For example, as the percentage of gas entering the production string from the well increases, the hydrostatic pressure in the production stream decreases. This temporary pressure fluctuation can create an unstable heading phenomena. For most cases, the un-choked simple orifice nozzles respond in an adverse manner by injecting more gas. On the other hand, the choked venturi nozzles of fixed geometry may inject the unchanged gas volume so long as the pressure differential is within a choked flow regime. However, to most effectively operate a gas lift system, the volume of gas injected from the injection side should be decreased in an equal amount to compensate for the increment of production gas in the production string. However, with a venturi of fixed geometry, the only efficient way to decrease the volume of gas entering the (production string) from the injection side is to reduce the throat dimensions of the valve. In current designs, changing the throat dimensions would mean removing the valve from the well and replacing it.
The opposite situation can also occur wherein the flow of gas into the tubing from the well deceases. To most efficiently operate the well in this instance, the volume of gas injected should be increased. However, as with the opposite scenario, increasing gas flow across a critical flow valve is impractical without changing the valve for one with a larger throat dimension.
Like gas lift, steam injection methods are known to increase the natural flow of production from a wellbore. In a steam injection scenario, wellbore(s) are created adjacent to or near a producing wellbore and steam is injected into these wellbores and permitted to exit and flow into a surrounding formation to heat hydrocarbons and urge them towards a nearby, producing wellbore. Venturi valves are commonly used in injection wells to maximize and stabilize the amount of steam injected from a tubing string into a formation. However, rather than controlling the volume of gas flowing from the annulus to the tubing string, the venturi valves in injection wells are used to control flow from the tubing to the annulus. Methods and apparatus for operating injection wells are taught in U.S. Pat. No. 6,708,763 owned by the assignee of the present application and that patent is incorporated herein in its entirety. Steam injection wells using venturi valves have the same limitations as gas lift wells in that the venturi is sized to permit a given volume of steam at a critical flow rate and increasing or decreasing that volume is impractical without changing the entire valve. There are times when an operator would like to increase or decrease the volume of steam delivered from a wellbore to a formation depending upon changes in natural conditions. The present arrangements make that impractical without changing equipment in the wellbore.
A need exists therefore for a critical flow valve which provides a simple way to increase or decrease gas or steam volume through the valve while maintaining a critical flow rate. A further need exists for a critical flow valve which is adjustable in order to adjust the size of the throat area of the valve. A further need exists for a venturi valve which includes an annular flow area defined by portions of plug and housing which are movable relative to each other between at least two positions, either position providing a different throat geometry.